Steam generator tubes of nuclear power plants are a heat exchanger which transfers heat from the primary coolant loop to the secondary one to produce steam in the latter. At an early stage of the nuclear industry, Alloy 600 was mostly used as steam generator tubes but with increasing plant operation time, Alloy 600 is well-known to be very susceptible to primary water stress corrosion cracking (PWSCC) (see Korean Laid-open Patent Publication No. 10-2010-0104928). To overcome this problem, Alloy 690 containing a higher content of Cr than Alloy 600 has recently been used as steam generator tubes, instead of Alloy 600, because Alloy 690 is well-known to be much higher resistant to PWSCC than Alloy 600.
Alloy 600 is a Ni-base alloy with a composition in weight percent of 14-17% Cr, 6-10% Fe, 0.15% C max, 1% Mn max, 0.5% Si max, 0.015% S max, and 0.5% by mass of Cu max, and Alloy 690 is a Ni-base alloy with a composition in weight percent of 27-31% Cr, 7-11% Fe, 0.05% C max, 0.5% Mn max, 0.5% Si max, 0.5% Cu max, and 0.015% S max.
As described above, Alloy 690 is a material with a higher Cr concentration than Alloy 600, which was called “Inconel Alloy 690,” after the name of the developer, or the Inco Alloys International. Inc. but is now called “Alloy 690” due to the expiration of the patent.